1. Field of the Invention
The invention pertains to a digital line scan camera
2. Description of the Related Art
Line scan cameras offer higher resolution than matrix cameras. Accordingly, satellite cameras are designed today almost exclusively as line scan cameras. Aerial cameras for aircraft are designed either as line scan or as matrix cameras. In a line scan camera, a complete picture is obtained by the relative movement between the line scan camera and the object, where the linear sensor is activated each time that movement by a certain distance has occurred. Typically this distance is equivalent to no more than one pixel. The complete image is then assembled from the totality of the linear image sections. So that the image sections can be assembled properly, it is necessary to know the external orientation of the individual scans, this orientation is defined by six parameters, 3 translation parameters and 3 rotation parameters. The internal orientation describes the location of the focal plane relative to the principal point of the objective lens and, as a result, characterizes a viewing direction in the camera coordinate system for each detector element. Once the internal and external orientations are known, a direct relationship can be established between the object points and the image points i.e., the pixels. An initially two-dimensional local coordinate can thus be assigned to object points identified in images. The knowledge of one coordinate such as the projection height, enables the correct determination of the two other coordinates in the world coordinate system (georeferencing).
One method of determining the external parameters with high precision is to combine a satellite-supported position determination device such as a GPS with a high-precision inertial navigation measurement unit (IMU=Inertial Measurement Unit). Strict requirements are imposed on the stability and relative accuracy of the measurements made by the IMU to determine the three angles. Stability here is a measure of the sensitivity of the system to low-frequency changes, whereas relative accuracy is a measure of higher-frequency changes. High stability is when the angular change during straight-ahead flight is less than ½° per hour. A high relative accuracy is present when the deviation is less than 1/100°. IMUs which fulfill both conditions are relatively expensive today, costing in the neighborhood of 150,000
IMUs are also known which have either high stability but low relative accuracy or low stability but high relative accuracy. Although these are comparatively inexpensive, they do not fulfill the accuracy requirements imposed on the angle determination of a location measurement system of a digital line scan camera.
A line scan camera comprising a focal plane, on which three linear sensors and four matrix sensors are arranged is disclosed in DE 38 02 219 A1. The matrix sensors are synchronously activated with each other, where the clock frequency is slower than that of the linear sensors. The orientation of the line scan camera is determined from the displacement between two successive images recorded by the matrix sensors. Two successive images recorded by the matrix sensors may not overlap each other too much; the overlap should not exceed 80% and should instead be in the area of approximately 60%. The reason for the overlaps is that it is impossible to distinguish between translational and rotational movements if the displacements are too small. It is impossible, in the case of a displacement by exactly one pixel, to determine whether a translation by one pixel has occurred or just a pitching movement of the camera. Thus the matrix sensors must cover an area large enough to ensure that they can detect a sufficiently large displacement.
Systems are also known which reference of the image data based on ground control points (GCPs), for which purpose three precisely measured control points are required.